Medical devices and sanitary goods that are not disposable should be disinfected or sterilized prior to the use in order to achieve their intended purpose. However, it is frequently observed that a medical device employed for a prolonged period of time does not sustain aseptic properties. Thus, it has been desired to overcome this problem.
The most effective solution to this problem has been to produce a medical device of which the surface contains a definite amount of an antimicrobial agent which is present throughout the use of the device. When such a device is prepared from a latex, it is required that the antimicrobial agent is homogeneously and stably blended with the latex which is then formed into the intended medical device.
It is known that heavy metals such as gold, silver, copper and zinc as well as compounds thereof exert an intense antimicrobial effect on a wide spectrum of microorganisms including various bacteria and fungi at extremely low concentrations of each metal ion. This effect is called an oligodynamic effect.
U.S. Pat. Nos. 4,483,688, 4,592,920, 4,603,152 and 4,054,139 disclose producing a medical device having an antimicrobial activity by taking advantage of the oligodynamic effect of these metals or compounds thereof. Namely, these metals or compounds thereof are dispersed in a matrix material which is then molded or used as a coating to produce the devices.
U.S. Pat. No. 4,483,688 discloses a catheter prepared by dispersing a fine metal powder such as silver powder in an appropriate binder and then coating a catheter with the same. However, this method is somewhat disadvantageous in the lack of complete dispersibility of said fine metal powder in the binder.
U.S. Pat. No. 4,592,920 discloses an antimicrobial catheter prepared by grinding an antimicrobial metal compound such as silver oxide to give a particle size of 30 .mu.m or less and dispersing the oxide in a suspension such as a latex or polyurethane which can then be hardened and formulated into a catheter. However, it is required in this case to add a surfactant to prevent the reaggregation of the metal compound particles during the grinding step as well as thereafter to maintain these particles in a stable state.
U.S. Pat. No. 4,603,152 discloses blending silver nitrate or silver oxide with a natural or synthetic rubber latex. In the Example 13 of this patent, a natural rubber latex is blended with powdery silver oxide. However, the dispersion of the powdery silver oxide in the highly viscous latex requires an undue prolonged period of time. Further, the addition must be carried out in a complex manner.
Further, U.S. Pat. No. 4,054,139 discloses coating the surface of a catheter with a polymer latex containing a silver compound such as silver nitrate. However, this method is disadvantageous in that the stability of the coating solution comprising the latex and the silver compound is poor, because the addition of silver nitrate to a latex causes to easily aggregate in the mixture. Thus, it is difficult to produce the coated catheter in a continuous manner.
In order to produce an antimicrobial medical device on an industrial scale, a suspension containing an antimicrobial agent and a mixture comprising a polymer which serves as a matrix, the antimicrobial agent and a solvent must remain stable during long term storage.
In general, metal compounds, especially silver compounds, are hardly soluble in water. When a silver compound is blended with a latex in the form of a solution, the silver concentration in the matrix material is extremely low. Thus, the required antimicrobial activity cannot be obtained in this case.
On the other hand, a latex such as a natural rubber latex dispersed in water is a highly unstable system. Thus, it gels even with a slight change in environmental factors such as pH or temperature. When an aqueous solution containing a silver compound such as silver nitrate which is highly soluble water is added to a latex at a high concentration in order to give a high silver concentration in a matrix material, silver nitrate would break the system wherein the latex suspended in the aqueous solution in a stable form. When silver carbonate which has an extremely low solubility in water is added, the stable latex dispersion system is also broken and aggregation is observed. Therefore, it has been impossible to obtain a stable latex composition.
Accordingly, a specific technique is required in order to prevent a stable latex dispersion system from breakage when a silver compound is dispersed in the latex and blended therewith.